Solar panels contain a plurality of solar cells comprising electrically connected solar wafers in an array which is then sealed between glass sheets. Current high efficiency solar panels have an initial output on the order of 20-22% cell efficiency (conversion of solar energy into electricity), but after about 2 weeks, drops off by over 10% into a typical range of 17-18%. However, the common solar panels made in the US have efficiencies that peak around 13% initially, and fall off well below that after being put into service use. This efficiency fall-off phenomena is called Light-Induced Degradation (LID). While the mechanism is not precisely known, research points to the correlation of Boron content to LID, it being postulated that Boron-Oxygen Complexes (BOCs) of uncertain empirical formula(s) are formed in the wafers upon exposure to sunlight. These complexes in the silicon are generally characterized as B5O2i as a result of post-production in-service-use exposure to light.
Among the highest initial solar cell conversion efficiencies are Boron doped MCz (Magnetic Czochralski) silicon wafers. Typically the Boron is doped into the silicon surface that later becomes the back contact surface as a result of coating (Printing) it with silver compound-containing paste which is then fired in a metallization furnace to form the silver back contact layer. However, the metallization does not prevent the LID of Boron after a few days to weeks of service, resulting in efficiency fall-off. To date there has been no process for preventing Boron-Oxygen complex LID of the solar cells, nor for their permanent regeneration back to original efficiency.
Amorphous silicon, α-Si, wafers experience LID on the order of 10-15% over their first 6-months of in-use service life. Crystalline silicon, c-Si, wafers experience LID on the order of 3-7%. In α-Si LID is also known as the Staebler-Wronski effect (SWE), which can stabilize at lower Solar Conversion Efficiency (SCE) levels after about 6-months of in-use service exposure to sunlight. In mono-crystalline silicon cells, the main cause of the LID effect is a recombination-active Boron-Oxygen complex. These complexes affect the carrier lifetime (rate at which the carriers recombine). The more B—O Complexes (BOCs) there are in the wafers, the faster the carriers recombine, reducing output. Experiments have been reported on exposure of solar cell wafers with light at 1 Sol intensity for several hours in an attempt to reduce LID effects. Such light exposure experiments have not been applied to industrial in-line wafer processing furnace systems. The results have not been satisfactory to date.
Accordingly, there is an unsolved need in the solar wafer processing field to produce wafers that do not exhibit efficiency degradation (LID), or exhibit a significant reduction of the amount or rate of LID, upon mounting as solar wafers as cells in solar panels that are put in service in the field. Further, any process that can reduce or prevent LID, or treat the wafer to regenerate it after metallization back to a stable initial efficiency state, must be solved in the context of being able to be retrofit into current wafer processing furnace apparatus, at production rates and yields that are economically feasible.